Telephone range extender with gain

ABSTRACT

The disclosure shows a telephone range extender which is automatically adaptable to the length of the connected subscriber loop. A loop resistance detector within the range extender discriminates between loops within a plurality of ranges of lengths. For extremely short loops not requiring range extension, the range extender circuits are disconnected from the loop. For intermediate length loops, the gain of a bidirectional voice frequency amplifier is set at a low value to provide just sufficient gain for these loops. For longer loops, the gain of the amplifier is increased to provide sufficient amplification for the longer range. For extremely long loops, the range extender is disconnected since such loops must be served by remote amplification facilities. An automatic range extender of this type is shown connected behind the first stage of the central office switch in order to concentrate range extenders on a larger plurality of subscriber loops.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to telephone subscriber loop range extenders and,more particularly, to a universal range extender applicable to anysubscriber loop and particularly suited for concentrated rangeextension.

2. Description of the Prior Art

In order to provide for telephone service on very long subscriber loops,it is often necessary to provide voice frequency gain and supervisorysignal detectors at the central office end of the subscriber loop. Suchspecialized facilities are called "range extenders with gain" (REGs).One such REG is shown in K. E. Stiefel U.S. Pat. No. 4,056,688, grantedNov. 1, 1977.

The need for a range extender on any particular subscriber loop isdependent on the length of that loop, normally specified by theresistance of the loop. Short loops, for example, do not require suchrange extension while very long loops cannot be serviced entirely fromthe central office end and usually require remote amplification. A rangeextender which is automatically adaptable to any length of loop isparticularly desirable since the loop parameters then need not beinvestigated prior to installation. The aforementioned Stiefel patentdiscloses one such automatically adjusting range extender in which thegain of the voice frequency amplifier is varied as a function of theloop length. The Stiefel range extender, however, is not suitable foruse with extremely long subscriber loops having remote amplificationfacilities.

Rather than providing a range extender for each subscriber loop in atelephone system, it is desirable to place the range extenders behindthe first stage of the central office switch, thereby providingconcentration of the subscriber loop traffic through the rangeextenders. Significantly fewer range extenders are required in thisarrangement. One system showing such concentrated range extensionfacilities is found in K. F. Giesken U.S. Pat. No. 3,816,668, issuedJune 11, 1974. The present invention is a range extender particularlysuitable for concentrated applications.

SUMMARY OF THE INVENTION

In accordance with the illustrative embodiment of the present invention,a range extender includes automatic loop resistance measuring apparatuswhich responds automatically to the length of the connected subscriberloop and provides appropriate voice frequency gain and supervisorysignal detection for the length of the particular connected loop. Moreparticularly, the range extender not only adjusts the gain in the voicefrequency path but includes means for automatically bypassing the rangeextender circuits for extremely short loops not requiring rangeextension and for extremely long loops which may include remoteamplification.

In accordance with one feature of the present invention, the rangeextender of the present invention is suitable for use on any subscriberloop in the telephone plant. This universal applicability makes itunnecessary to determine the parameters of a particular subscriber loopprior to installation of the range extender. Moreover, only a singlerange extender need be kept in stock which can be used wherever rangeextension is desirable.

In accordance with another feature of the present invention, theuniversal automatically adjusting range extender of the presentinvention can be used in concentrated applications. That is, the rangeextension circuitry can be inserted behind the initial stages of thecentral office switch and thereby be used to serve more than onesubscriber per range extender. Significant savings in range extensioncircuitry are thereby effected, relying on the automatic adjustment ofthe range extender to the particular subscriber loop connected to therange extender at any particular time.

A further feature of the present invention is the ability to disconnectthe range extension circuitry immediately prior to a switching operationof the central office switch. This permits the central office switchingcrosspoints to be operated with no signals on the central office switchconductors, i.e., the central office switch is operated "dry."

The major advantage of the range extending circuitry of the presentinvention is its universal applicability to all situations requiringrange extension, including those situations where it is desired toconcentrate the range extenders within the central office switch.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a general block diagram of a range extender with gain inaccordance with the present invention;

FIG. 2 is a detailed circuit diagram of a variable gain bidirectionalamplifier useful in the range extender shown in FIG. 1;

FIG. 3 is a detailed circuit diagram of a loop voltage sensor useful inthe range extender of FIG. 1;

FIG. 4 is a detailed circuit diagram of a ringing signal detector usefulin the range extender of FIG. 1;

FIG. 5 is a detailed circuit diagram of a central office voltagedetector useful in the range extender of FIG. 1;

FIG. 6 is a detailed circuit diagram of a three-level resistancedetector useful in the range extender of FIG. 1;

FIG. 7 is a detailed circuit diagram of a common logic circuit useful inthe range extender of FIG. 1;

FIG. 8 is a detailed circuit diagram of a power supply useful in therange extender of FIG. 1; and

FIG. 9 is a general block diagram of the manner in which rangeextenders, such as that shown in FIG. 1, can be placed inside of atelephone central office switch to provide shared range extension for alarge plurality of telephone subscriber loops.

DETAILED DESCRIPTION

Referring more particularly to FIG. 1, there is shown a range extenderconnected between the central office appearances of tip conductor 10 andring conductor 11 and the subscriber loop appearances of tip conductor36 and ring conductor 13. A ringing signal detector 14 is connectedbetween tip conductor 10 and ring conductor 11 at the office side of therange extender. Similarly, a central office voltage detector 12 isconnected between tip conductor 10 and ring conductor 11. The outputs ofdetectors 12 and 14 are applied to a common logic circuit 15 whichresponds to these outputs and to various other signal conditions tocontrol the operation of a line (L) relay 16 and a range extenderoperate (RO) relay 17. When operated, L relay 16 closes L contacts 18 toconnect a load resistor 19 between tip conductor 10 and ring conductor11. When operated, RO relay 17 operates transfer contacts 20, 30 and 32to connect the range extender circuits into the loop between conductors10-11 and conductors 36-13.

Power supply 21 is operated from the central office voltage andgenerates supply voltages of the proper magnitudes to operate theelectronic circuits in the bilateral amplifier 22. Tip conductor 10 andring conductor 11 are connected through power supply 21 to bilateralamplifier 22. Amplifier 22 provides amplification of voice frequencysignals traveling in either direction and can provide such amplificationat either one of two gain levels, under the control of a signal onautomatic gain control (AGC) lead 23. The right-hand signal path fromamplifier 22 is connected through a compensating impedance 24 to theprimary winding 25 of transformer 26.

Transformer 26 has two secondary windings 28 and 29 separated by acapacitor 27. One end of winding 28 is connected to ground potentialwhile the other end is arranged to be connected by RO transfer contacts30 to the subscriber tip conductor 36. Similarly, winding 29 has one endconnected to a negative voltage source 31 which may, for example,comprise a boosted battery supply, and the other end connected throughRO transfer contacts 32 to ring conductor 13 of the subscriber loop. Itcan be seen that RO contacts 30 and 32 switch subscriber loop conductors36 and 13 from a direct connection to central office conductors 10 and11, respectively, to the bilateral amplification path including powersupply 21, amplifier 22, impedance 24 and transformer 26.

A loop voltage sensor 33 is connected between tip conductor 36 and ringconductor 13. A loop current sensor 34 is coupled to loop conductors 36and 13. Sensors 33 and 34 provide outputs to a three-level resistancedetector 35. A current sensor suitable for use as sensor 34 is shown inthe copending application of J. L. Henry, Ser. No. 61,463, filed July27, 1979. Using the indication of line voltage from sensor 33 and theindication of loop current from sensor 34, resistance detector 35distinguishes between three separate and distinct levels of subscriberloop resistance, for convenience, identified as a high, a medium, and alow resistance threshold. Control signals indicating loop resistancescrossing these thresholds are supplied to common logic circuit 15.

The range extender shown in block form in FIG. 1 operates as follows.When connected to a particular subscriber loop (leads 36 and 13), therange extender measures the loop resistance (via sensors 33 and 34 anddetector 35). L relay 16 and RO relay 17 are operated in response tosuitable loop resistance between conductors 36 and 13. The operation ofRO relay 17 indicates the subscriber has gone off-hook and henceamplification of the voice signals is required. L relay 16 follows dialpulses, represented by interrupted current flow in conductors 36 and 13,and repeats these dial pulses to the central office by terminatingcentral office conductors 10 and 11 with the amplifier 22 in approximatesynchronism with dial pulses. For subscriber loops having a resistanceexceeding a medium threshold (TM), logic circuit 15 produces a signal oncontrol lead 23 to shift the gain of amplifier 22 from 3 decibels to 6decibels. For loop resistances exceeding a high threshold (TH), it isassumed that the subscriber loop is an open circuit (ON-HOOK) and both Lrelay 16 and RO relay 17 are released. For loop resistances less than alow threshold (TL), it is assumed that the subscriber loop issufficiently short that range extension is not necessary. Under thiscondition, L relay 16 and RO relay 17 are likewise released and signalscan flow between conductors 10 and 11 and conductors 36 and 13 withouttraversing the amplification path provided by amplifier 22.

The importance of the arrangement shown in FIG. 1 is readily apparent ifit is assumed that the range extender is connected behind the firststage or stages of switching in the telephone central office, as shownin FIG. 9. When used in this way, it is not known what length ofsubscriber loop will be connected to the range extender at any giventime. The range extender is therefore constructed to automaticallyrespond to the measured loop resistance by providing an appropriateamplifier gain or, for short loops, to bypass the range extenderaltogether. With the concentration provided by the central officeswitch, it is unnecessary to provide a range extender for eachsubscriber loop. Significant economies are obtained by thusconcentrating the range extension function within the central officeswitch.

In FIG. 2 there is shown a detailed circuit diagram of the bidirectionalamplifier 22 of FIG. 1. The amplifier in FIG. 2 comprises twounidirectional differential amplifiers 50 and 51 each having both apositive and a negative input and a single output. Voice frequencysignals arrive from opposite directions on tip conductor 52 and tipconductor 53, respectively, relative to common ring conductor 54. Thenegative input to amplifier 51 is connected through resistive-capacitive(RC) impedance 55 to tip conductor 52. Similarly, tip conductor 53 isconnected through direct current isolating capacitor 80 and RC impedance56 to the negative input of amplifier 50. The output of amplifier 50 isconnected through RC impedance 57 to tip conductor 52. The output ofamplifier 51 is connected through RC impedance 58 to tip conductor 53.The output of amplifier 50 is also connected through an RC voltagedivider network 59 and 60 to the positive input of amplifier 51. Theoutput of amplifier 51 is connected through RC voltage divider network62 and 63 to the positive input of amplifier 50. The local ground forthese voltage dividers is provided from FIG. 8, to be describedhereinafter.

A pair of negative feedback resistors 65 and 66 are connected in seriesfrom the output of amplifier 50 to the negative input of amplifier 50.The junction of resistors 65 and 66 is connected through resistors 67and 68 to ground potential. A field effect transistor (FET) 69 has itsmajor electrodes connected in shunt across resistor 68. The controlelectrode of FET 69 is connected to the emitter of light detectingtransistor 70.

Negative feedback resistors 71 and 72 are connected in series from theoutput of amplifier 51 to the negative input of amplifier 51. Thejunction of resistors 71 and 72 is connected through resistors 74 and 75to ground potential. The major electrodes of FET 76 are connected inshunt across resistor 75. The control electrode of FET 76 is connectedto the emitter of photodetecting transistor 70.

The collector of photo-detecting transistor 70 is connected to positivevoltage source 77 while the emitter of transistor 70 is connectedthrough resistor 78 to ring conductor 54, which is also connected tonegative voltage source 79. A current appearing on AGC lead 23 isapplied through resistor 81 and light-emitting diode (LED) 82. Diode 82emits light which is detected by transistor 70 to enable thecollector-emitter path of transistor 70. The voltage drop acrossresistor 78 provides a sufficient voltage to enable FETs 69 and 76.Resistors 68 and 75 are thereby effectively removed from the feedbackcircuit and the voltage divisions in the negative feedback paths aroundamplifiers 50 and 51 are modified to provide a smaller feedback signaland hence more gain (6 decibels instead of 3 decibels).

In operation, the signals appearing at tip conductors 52 and 53 arebidirectional. The signals derived from amplifier 50 represent thesignals traveling from right to left in the circuit. Signals originatingat tip conductor 52, traveling from left to right, traverse impedance55, amplifier 51, impedance 58 and capacitor 80 to tip conductor 53. Aportion of the output of amplifier 50 is applied through voltagedividing network 59 and 60 to the positive input of amplifier 51 and isthereby subtracted from the composite signal supplied through impedance55. The output of amplifier 51 is therefore the difference between thesesignals and represents only the signal traveling from left to right.

Similarly, the signals at tip conductor 53 are applied through capacitor80 and impedance 56 to the negative input of amplifier 50. A portion ofthe output of amplifier 51 is applied through voltage dividing network62 and 63 to the postive input of amplifier 50 and is thereby subtractedfrom the composite signal at the negative input of amplifier 50. Theoutput of amplifier 50 is therefore only the signal traveling from rightto left, as originally assumed.

The gains of amplifiers 50 and 51 are controlled by the negativefeedback paths from their respective outputs to their respectivenegative inputs. With FETs 69 and 76 in the enabled condition, thesegains are set at approximately 6 decibels. When FET switches 69 and 76are disabled by loss of current on AGC leads 23, the gain of amplifiers50 and 51 is changed so as to provide a 3 decibel gain. In this way, theoverall gain provided by the amplifier configuration of FIG. 2 can beadjusted to one of two different levels depending on the resistance ofthe connected telephone loop.

In FIG. 3 there is shown a circuit diagram of the loop voltage sensor 33of FIG. 1. The voltage sensor of FIG. 3 comprises a differentialamplifier 120 having a positive and a negative input. The tip conductor36 is connected via voltage divider 121-123 to the positive input ofamplifier 120. The ring conductor 13 is connected through resistor 122to the negative input of amplifier 120. A negative feedback resistor 124is connected from the output of amplifier 120 on lead 125 to thenegative input of amplifier 120.

In operation, the voltage on tip conductor 36 is applied to the positiveinput of amplifier 120. The voltage on ring conductor 13 is appliedacross the voltage divider comprising resistors 122 and 124 to thenegative input of amplifier 120. Resistor 124 provides negative feedbackwhich determines the amplifier gain. The output of amplifier 120 on lead125 is linearly proportional to the voltage between conductors 36 and 13and has a polarity corresponding to the polarity of the voltage (ringvolts with respect to tip volts) between conductors 36 and 13 within thevoltage swing permitted by the amplifier and its power supplies.

In FIG. 4 there is shown a ringing signal detector which may be used asringing detector 14 in FIG. 1. In FIG. 4, tip conductor 10 is connectedthrough a direct current blocking capacitor 150, across LEDs 154 and155, and resistor 153 to ring conductor 11. Capacitor 150 serves toisolate the ringing detector from direct current voltages. The currentflow through the two oppositely poled light-emitting diodes (LEDs) 154and 155 in the presence of a ringing signal causes LEDs 154 and 155 tobe energized. These diodes emit visible light which is detected byphoto-sensitive transistor 156. When enabled by a light input, thecollector-emitter path of transistor 156 is enabled to provide adifferent voltage drop across resistor 157. This voltage is applied toNAND gate 158 together with a signal on lead 159 which indicates that ROrelay 17 is not operated. NAND gate 158 is therefore fully enabled onlywhen a ringing signal is present on leads 10 and 11 and the RO relay isnot operated, indicating that the range extension circuits have not beenswitched into the subscriber circuit and that central office ringing hascommenced. The output of NAND gate 158 appears on lead 160 and SCD lead161. As will be described in connection with FIG. 8, this signal is usedto modify the loop current detection circuitry in the presence ofringing signals for the purpose of more readily detecting ring-tripcurrents.

In FIG. 5 there is shown a detailed circuit diagram of the centraloffice voltage detector 12 of FIG. 1 which is connected between tipconductor 10 and ring conductor 11. Tip conductor 10 is connectedthrough resistor 170 to the negative input of differential amplifier171, while ring conductor 11 is connected to a voltage dividercomprising resistors 172 and 173. The junction of resistors 172 and 173is connected to the positive input of amplifier 171. Feedback resistor174 is connected from the output of amplifier 171 to the negative input.As previously discussed in connection with FIG. 3, amplifier 171provides an output signal linearly related to the voltage betweenconductors 10 and 11, both in magnitude and in polarity.

The output of amplifier 171 is connected through resistors 175 and 176to positive voltage source 177. The output of amplifier 171 is alsoconnected through resistor 178 and inverter circuit 179 to one input ofNAND gate 180. The junction of resistors 175 and 176 is connected to theother input of gate 180. Resistors 175 and 176 and gate 180 establish avoltage threshold referenced to leads 10 and 11 of about 10 volts.Similarly, resistor 178 and inverter 179 establish a voltage thresholdof about 10 volts for the other polarity of central office voltageappearing across conductors 10 and 11. The output of NAND gate 180 istherefore a binary or two-state output on lead 181 which indicates thatthe ±10-volt threshold level between conductors 10 and 11 has beenexceeded. This threshold is set to detect the loss of the normal centraloffice battery voltage which occurs just prior to the operation of thecentral office switching crosspoints. In applications such as that shownin FIG. 9, all voltages are removed from the tip and ring conductorsprior to each switching operation so that the switching elements do nothave to interrupt or initiate current flows as they are operated (i.e.,they are operated "dry"). The output of the detector in FIG. 5 on lead181 is therefore used to rapidly release both L relay 16 and RO relay17, thereby removing the range extension circuitry from the circuitduring central office switching. The manner in which this isaccomplished will be discussed in more detail in connection with FIG. 7.

In FIG. 6 there is shown a detailed circuit diagram of a three-levelloop resistance detector. The resistance detector of FIG. 6 responds tothe loop current sensor 34, shown in FIG. 1, the output of which appearson conductor 116 in FIG. 6, and the output of the loop voltage sensor ofFIG. 3, the output of which appears on lead 125 in FIG. 6. The detectorof FIG. 6 comprises three differential amplifiers 200, 201 and 202. Eachof these amplifiers is arranged to detect a different resistance levelfor either polarity of central office battery voltage. Amplifier 200,for example, is designed to detect loop resistances which are less than1700 ohms (the low resistance threshold). Amplifier 201 is arranged todetect loop resistances that exceed a high threshold of 4300 ohms andamplifier 202 is arranged to detect resistances exceeding a mediumthreshold of 2200 ohms.

The loop voltage signal appearing on input lead 125 is applied across avoltage divider including resistors 203 and 204 to ground potential. Thejunction of resistors 203 and 204 is connected to the negative input 206of amplifier 200. The loop current signal on lead 116 is similarlyconnected across a voltage divider including resistors 207, 208 and 209to ground potential via FET 210 or FET 211. The junction of resistors207 and 208 is connected to the positive input 213 of amplifier 200. Itcan be seen that the operation of FET 210 or FET 211 alters the voltagedivision ratio, thereby changing the portion of the signal on lead 116which is applied to input 213 to obtain a threshold hysteresis which isunder the control of the logic circuit. The output of amplifier 200 isapplied through resistor 214 to output lead 215.

In operation, amplifier 200 serves as a comparator which compares thevoltages at inputs 206 and 213. The resistance of the connectedtelephone loop is the ratio of the loop voltage on lead 125 and the loopcurrent on lead 116. The threshold occurs for amplifier 200 when thesignals on leads 206 and 213 are approximately equal. It can easily beseen that the threshold of operation of amplifier 200 is controlled bythe voltage division ratios and, moreover, is independent of themagnitude or polarity of the actual loop voltage. This property isimportant for subscriber loops which may have differing office voltagesapplied to them, such as normal office voltage (-48 volts), reversedvoltages and boosted voltages (-78 volts). At midrange and higherresistance values, amplifier 200 is not activated. If the loopresistance falls below the predetermined threshold (1700 ohms),amplifier 200 operates to provide a change in output signal on lead 215.

Amplifier 201 operates in essentially the same manner but with importantdifferences due to the FET circuits being connected to the voltage inputrather than the current input as was the case for amplifier 200. Thesignal indicating loop voltage on lead 125 is applied through a voltagedivider including resistors 216, 217 and 218 to ground potential via FET224 or FET 225. The junction of resistors 216 and 217 is applied to thenegative input 220 of amplifier 201. The signal indicating loop currenton lead 116 is applied through a voltage divider including resistors 221and 222 to ground potential. The junction of resistors 221 and 222 isapplied to the positive input 223 of amplifier 201. FETs 224 and 225 areused to control the voltage division ratio of signals applied to thenegative input 220 of amplifier 201. The output of amplifier 201 isapplied through resistor 224 to output lead 225.

Amplifier 201 changes output state when the loop resistance crosses theupper threshold (4300 ohms). The thresholds of both amplifier 200 andamplifier 201 are modified by means of control signals on leads 226 and227, generated when RO relay 17 or L relay 16 is operated, as will bedescribed in connection with FIG. 7. The signal on lead 226 is appliedacross a voltage divider including resistors 228 and 229 to positivevoltage source 230. The junction of resistors 228 and 229 is connectedto the control electrodes of FETs 210 and 224. Similarly, the controlsignal on lead 227 is applied to a voltage divider including resistors231 and 232 to a positive voltage source 233. The junction of resistors231 and 232 is connected to the control electrodes of FETs 211 and 225.

In operation, the control signals on leads 226 and 227, in combinationwith FETs 210, 211, 224 and 225, alter the thresholds of amplifiers 200and 201 to provide hysteresis in the threshold characteristic. That is,once amplifier 200 or amplifier 201 has changed its output state andhas, in turn, operated L relay 16 or RO relay 17, the thesholds areadjusted so that these amplifier outputs will not change back to theformer state until the input resistance reaches a different threshold.This prevents the amplifier outputs from cycling between high and lowstates for loop resistances near the threshold values.

A gate-by-gate explanation of the logic circuit operation is givenlater; a brief abstract is presented here to illustrate the overalloperation. Since the polarities of the loop voltage and loop current areimportant to successful operation and since, for a given threshold,i.e., TH, only one amplifier (such as amplifier 201) is used to providethe threshold for dual-polarity detection, logic circuits connecting tothe amplifier process the amplifier output along with inputs which comefrom the loop voltage detector amplifier 120. The logic circuit outputto resistors 305 and 317 is low for either polarity of voltage which isgreater than about 10 volts as seen at the loop voltage detector whenthe loop resistance is less than the threshold level of the amplifier201. The following table summarizes the loop voltage detector and loopcurrent detector inputs to the TH amplifier and the logic circuit andshows the output logic level for each set of inputs.

                  TABLE                                                           ______________________________________                                                              Sign of        Logic                                                Loop      TH Amplifier   Output                                   Loop Voltage                                                                              Current   Input 223      to R305                                  Sign Magnitude  Sign      (Ref = Input - 220)                                                                        & R317                                 ______________________________________                                        -    > 10V      -         +            HIGH                                   -    > 10V      -         -            LOW                                    +    > 10V      +         -            HIGH                                   +    > 10V      +         +            LOW                                    -    < 10V      -         +            HIGH                                   -    < 10V      -         -            HIGH                                   +    < 10V      +         -            HIGH                                   +    < 10V      +         +            HIGH                                   -    > 10V      +         +            HIGH                                   +    > 10V      -         -            HIGH                                   ______________________________________                                    

A dial pulse detection circuit is connected across resistor 221. Acapacitor 235 is connected in parallel with resistor 221 by the contact236 of RO relay 17 (FIG. 1). In operation, RO relay 17, and hence ROcontact 236, operates in response to an off-hook signal received fromthe subscriber end of the loop. Dial pulses are represented bytransitions between loop resistances of a low value (closed circuit) andloop resistances of a high value (open circuit). However, since thecapacitance of the telephone loop must be charged or discharged on eachdial pulse transition, the apparent loop resistance rises rather slowlywhen the dial contacts open and the line capacitance is charged. Anoscillatory effective resistance is produced when the dial contacts openthe loop and the line also charges the ringer circuit. In order toaccurately track dial pulses, it is therefore desirable to provide ahigh resistance threshold when the dial contacts close the loop and amuch lower threshold when the dial contacts open the subscriber loop.This variable threshold provides more accurate detection of thetransition times and avoids false indications (split pulses) due tooscillatory peaks in the line resistance by providing a controlled rateof transition from one threshold to the other which masks the ringercharging current oscillations. Capacitor 235 is charged prior to thedial contact opening and thus provides the lower resistance thresholdfor amplifier 201. When the dial contact closes, capacitor 235 chargesthrough resistor 222 to provide the higher resistance threshold foramplifier 201. Thus, when the dial pulse terminates, a new and higherthreshold is provided to amplifier 201 and the dial pulse's terminationis detected at a different threshold than the dial pulse initiation.Moreover, the shift in threshold is proportional to the loop current,thus tending to track the dial pulse amplitude.

Amplifier 201 is also used to provide an indication of ring trip. Thatis, amplifier 201 must detect the current flow due to the operation ofthe subscriber's switchhook when he goes off-hook in response to ringingsignals. This requires a relatively small direct current to be detectedin the presence of the alternating high voltage and high current 20 Hzringing signals. When ringing occurs, as indicated by ringing detector14 (FIG. 1), the inhibit signal on lead 160 (FIG. 6) is removed fromacross resistor 239 and the control electrodes of FETs 240 and 251. FET240 therefore operates to connect a capacitor 241 from positive input223 of amplifier 201 to ground potential. Capacitor 241 filters out thelarge varying alternating current component on the subscriber loopcaused by the ringing signals but slowly builds up a net charge due tothe direct current signal, thereby permitting amplifier 201 and logiccircuits (FIG. 7) to detect the small direct current off-hook signalwith considerably greater sensitivity. A resistor 242 is connectedacross capacitor 241 to discharge capacitor 241 when it is not in use.FET 251 connects resistor 250 from input 220 of amplifier 201 to ground.This shifts the detection threshold to account for the lowersuperimposed direct current (38 volts versus 48 volts).

The ring-trip detection threshold is further defined by the design ofthe dynamic range of the loop voltage detector so that during ringingthe output of the loop voltage amplifier 120 clips the peak of thelarger (absolute magnitude) 20 Hz voltage crest. The clipping at theextremities of the loop voltage amplifier 120 output therefore providesa reference voltage for a portion of each 20 Hz cycle to measure thevoltage output of the loop current detector against. Ring-tripdetection, as a result, is a current threshold measurement rather than aresistance threshold measurement. The TH amplifier 201 and the logiccircuits supply drive to resistors 305 and 317 when the dc component ofthe ringing circuitry exceeds the threshold of the amplifier 201 for afraction of each 20 Hz cycle. Filtering in the RO relay logic circuitdelays the RO relay operation. The L relay operation is fast enough tofollow the LOW logic level inputs to its driver. The L relay operatesduring each high level voltage peak of the ringing voltage waveform. Itfollows then that the periodic L contact closures selectively aid theclosure of the loop by causing rectified high current pulses to flowthrough diode 38 (FIG. 1) that reenforce the loop ring-trip signal. Thelogic circuits act, as detailed in the table, to track the instantaneouspolarity of loop voltages during the ringing intervals to make ring-tripdetection possible for either polarity of ring-trip battery when appliedto either tip or ring conductors.

The loop current signal on lead 116 is also connected directly to thenegative input 250 of amplifier 202. The loop voltage signal on lead 125is applied through a voltage divider including resistors 251, 252 and253 to ground potential. The junction of resistors 252 and 253 isconnected to the positive input 254 of amplifier 202. The output ofamplifier 202, appearing on lead 23, is connected through feedbackresistor 256 to the junction of resistors 251 and 252. The signal onlead 23 controls the automatic gain control circuits of FIG. 2. Thepositive feedback through resistor 256 provides hysteresis and preventsundesirable gain changes caused by slight loop resistance fluctuation or60 Hz noise pickup.

The detector of FIG. 6 detects subscriber loop resistances in fourdifferent ranges separated by three different thresholds. The highestrange, above the highest threshold (TH) of amplifier 201 (>4300 ohms),indicates an open circuit and is used to detect the off-hook or dialpulse condition. The next lower range, between the medium threshold (TM)of amplifier 202 and the high threshold (2200 through 4300 ohms),indicates a long subscriber loop which requires a 6 decibel gain in theamplifier of FIG. 2. The next lower range (1700 through 2200 ohms)indicates an intermediate length loop requiring a 3 decibel gain in theamplifier of FIG. 2. The lowest range (below 1700 ohms) indicates a loopwhich is short enough so as not to require range extension at all. Inthis case, L relay 16 and RO relay 17 are both released and the rangeextender circuits are disconnected from the subscriber loop. The highthreshold (amplifier 201) is also used for ring-trip and dial pulsedetection.

In FIG. 7 there is shown a detailed circuit diagram of the common logiccircuit 15 of FIG. 1. The output of the central office voltage detectorof FIG. 5 is supplied on lead 181 to the circuit of FIG. 7. The outputof the high resistance threshold detector 201 of FIG. 6 is applied onlead 225. The output of the low resistance threshold detector 200 ofFIG. 6 appears on input lead 215. The output of the loop voltage sensorof FIG. 3 appears on input lead 125.

The signal on lead 225 from the high threshold resistance detector isapplied through inverter circuit 300 to NAND gate 301 and is applieddirectly to NAND gate 302. The outputs of NAND gates 301 and 302 areapplied to NAND gate 303, the output of which is applied to NAND gate304. The output of NAND gate 304 is applied to a voltage dividercomprising resistors 305 and 306, the junction of which is connected tothe base of transistor 307. Transistor 307 has its emitter connected topositive voltage source 308 and its collector connected through the coilof L relay 16 to negative voltage source 309. A capacitor 310 isconnected between the base and collector of transistor 307.

The output of NAND gate 303 is also applied through a delay circuitcomprising diode 342, resistor 343 and capacitor 346 to inverter 341.The input to inverter 341 is normally biased from negative source 344through resistor 345 to provide a negative input to inverter 341. Thepositive output of inverter 341 is again inverted by inverter 347 andapplied to NAND gate 316. NAND gate 316 therefore cannot be fullyenabled until capacitor 346 is charged positive by the output of NANDgate 303 when the high threshold is reached. This insures a delay in theoperation and release of RO relay 17, thereby holding RO relay 17operated as L relay 16 tracks dial pulses.

The signal on lead 215, representing the low resistance threshold, issupplied directly to NAND gate 311 and through inverter 312 to NAND gate313. The outputs of NAND gates 311 and 313 are applied to NAND gate 314,the output of which is connected through inverter 315 to NAND gate 316and also comprises one input to NAND gates 301 and 302. The output ofNAND gate 316 is applied to a voltage divider including resistors 317and 318, the junction of which is connected to the base of transistor319. The emitter of transistor 319 is connected to positive voltagesource 320 while the collector of transistor 319 is connected throughthe coil of RO relay 17 to negative voltage source 321. A capacitor 322is connected from the base to the collector of transistor 319.

An output from NAND gate 314 indicates that the line resistance is belowthe lower threshold. This signal is inverted in inverter 315 and used todisable RO relay 17 (via NAND gate 316) and to disable L relay 16 (viaNAND gates 301 and 302). Both L relay 16 and RO relay 17 are alsodisabled by the signal on lead 181, indicating that the office voltagehas been removed and hence switching of the line is about to take place.Relays 16 and 17 are therefore released to permit "dry" switching by thecentral office crosspoints.

The loop voltage signal on lead 125 is connected across a voltagedivider comprising resistors 323 and 324 to positive voltage source 325.The junction of resistors 323 and 324 is connected through inverter 326to the remaining input of NAND gate 301 and to the remaining input ofNAND gate 313. The loop voltage signal on lead 125 is also suppliedthrough resistor 327 to the remaining input of NAND gate 302 and to theremaining input of NAND gate 311.

It will be recalled that the sensors 33 and 34 of FIG. 1 provide linearoutputs representing voltage and current, respectively, on thesubscriber loop. Moreover, these voltages and currents can be of eitherpolarity. The threshold detectors of FIG. 6 are therefore also arrangedto respond to signals of either polarity and to produce outputs thesignificance of which is inverted when the polarity of the inputs isinverted. In order to avoid the necessity of providing separatethreshold detectors for these opposite polarities, the logic of FIG. 7is adjusted by a polarity-indicating signal on lead 125. If the linevoltage is positive, for example, NAND gates 302 and 311 are enabled viaresistor 327. If the line voltage is negative, NAND gates 301 and 313are enabled via inverter 326. In this way, the NAND gates 303 and 314respond correctly to the magnitude indication on leads 225 and 215,respectively, regardless of their polarity. For positive magnitudes,NAND gates 303 and 314 are operated through NAND gates 302 and 311,respectively, while for negative magnitudes, NAND gates 303 and 314 areoperated through NAND gates 301 and 313, respectively. Since theadditional logic is much cheaper and more compact than would be theadditional threshold detectors, the arrangement shown in the Figures ispreferable.

Additional inputs to gates 311 and 313 from SCD lead 161 inhibit theeffects of signals on lead 215 from TH amplifier 200 from interferingwith ring tripping. Without the inhibit function, high level ringingcurrents could be detected by the TH amplifier and the logic circuitwould then act to prevent the L and RO relays 16 and 17 from operatingfollowing the TH set off-hook response to ringing because the THamplifer does not have a switchable low-pass filter that is similar tothe filter on the input of the TH amplifier that removes the 20 Hzcurrent.

A signal on lead 328, when appearing in combination with a positiveoperate voltage on lead 329, operates transistors 330 and 331 to providean output signal through resistor 332 and inverter 333 to one input ofNAND gate 304 and to one input of NAND gate 316. The signals on leads328 and 329 permit the telephone switching machine to disable relays 16and 17 independent of the line conditions. This relay control is usedfor one type of concentrated range extension where the switching machineitself detects all dial pulsing and ring tripping while the rangeextender is held in a bypass condition. Other types of switchingmachines do not have this capability and the range extender isautomatically released via detector 12 and lead 181.

The office voltage signal on lead 181 is applied to one input of NANDgate 304 and to one input of NAND gate 316 to disable relays 16 and 17if office voltage is lost. The signal on lead 181 is also appliedthrough inverter 334 and resistor 335 to the input of inverter 336. Theinput to inverter 336 is biased from positive voltage source 337 throughresistor 338 and is connected to ground potential through capacitor 339.The output of inverter 336 is supplied through resistor 340 to capacitor346. The signal on lead 181 therefore discharges capacitor 346 to resetthe delay timing circuit. As previously noted, the output of inverter341 is applied through inverter 347 to one input of NAND gate 316. Theoutput of inverter 341 also comprises the control signal on lead 159used to enable the output of the ringing detector of FIG. 4. The outputof inverter 341 is also combined with the output of NAND gate 304 inNAND gate 348. The output of NAND gate 348 is connected so as to rapidlydischarge capacitor 346, thus resetting the timing delay circuit in theoperate circuit of relay 17.

Leads 226 and 227 provide hysteresis for the loop resistance detectorfunctions of the TL and TH comparators 200 and 201 of FIG. 6. Hysteresischanges the magnitude of the resistance thresholds so as to providenonpolarity sensitive feedback around the loop consisting of the TH (andTL) comparator, the logic and relays of FIG. 7, the actual loop cableimpedance and sensors 33 and 34. With nonpolarity sensitive feedback,the TH comparators will require an approximately 50 percent change inloop resistance after having switched to one state before it will switchback. This hysteresis makes the circuit insensitive to noise or cabletemperature changes.

In FIG. 8 there is shown a detailed circuit diagram of the power supply21 of FIG. 1. Tip conductor 400 and ring conductor 401 are connected toa full-wave rectifier (polarity guard) 402. Across the output of guard402 is a series circuit including resistor 403, zener diode 404 andcapacitors 405 and 406. The junction between resistor 403 and zenerdiode 404 is connected through resistor 407 to the base of a Darlingtonpair of transistors 408 and 409. A capacitor 410 is connected from thebase of transistor 408 to the junction of zener diode 404 and capacitor405. The emitter of transistor 409 is connected through a voltagedivider comprising resistors 411, 412 and 413 to the negative voltagesupply 179. The collectors of transistors 408 and 409 are connected tolead 415. Voice frequency signals are connected to the bilateralamplifier 22 of FIG. 1 by way of leads 415 and 179.

In operation, the rectifier 402 translates the direct current voltagesappearing at the central office appearances 10 and 11 (FIG. 1) into aunidirectional voltage having a polarity positive on lead 415 andnegative on lead 179. Zener diode 404 provides a reference voltage tobias transistors 408 and 409 to a preselected operating point whichdelivers a current of known magnitude to the voltage divider includingresistors 411, 412 and 413. Resistor 407 and capacitor 410 remove voicefrequency alternating current components from the control signaldelivered to the base of transistor 408. Capacitors 405 and 406 smooththe direct current voltages appearing across their terminals. Thejunction of resistors 411 and 412 provides a positive voltage on lead416 which can be used as a positive power supply for the bidirectionalamplifier of FIG. 2. The midpoint of resistors 412 and 413 is groundedto a local ground, and the voltage on lead 179 acts as a megativevoltage supply for the bidirectional amplifier. It will be noted thatpolarity guard 402 is biased heavily "on" and therefore provides a verylow impedance path for voice frequency signals from leads 400 and 401 toleads 179 and 415. The power supply of FIG. 8 powers only thebidirectional amplifier of FIG. 2. The balance of the circuits ispowered from the central office battery.

In FIG. 9 there is shown a block diagram of the manner in which rangeextenders of the form shown in FIG. 1 can be connected behind the firstor concentrator switching stages of a central office switch. Thus, rangeextenders 450 are connected between switching stage 451 and later stagesof switching (not shown) to provide concentrated range extension for thesubscriber loops 452. Since range extenders 450 automatically adapt toloops of varying resistances, it is possible to connect a wide varietyof subscriber loops 452 to switch 451. Moreover, since switch 451concentrates traffic from loops 452, a smaller number of range extenders450 are required with the arrangement of FIG. 11 than would be requiredif each of subscriber loops 452 were separately equipped with a rangeextender. It is to be understood, however, that individual ones of loops452 could nevertheless be equipped with a range extender of the formshown in FIG. 1. Individually connected range extenders wouldautomatically adapt to the loops to which they were connected. It istherefore apparent that the range extender of the present invention isuniversally applicable in the telephone plant and is particularly usefulwhen used behind the early switching stages of a central office switch.Moreover, in view of the range extender's ability to rapidly disconnectduring the switching operation, the range extender of the presentinvention is also suitable for use in electronic central officeswitches.

We claim:
 1. A telephone subscriber loop range extender includingcircuits for enhancing telephone service over subscriber loops longerthan average in length, said range extender characterized byfirst meansfor detecting the length of a connected subscriber loop, and meansresponsive to said first detecting means for disconnecting saidenhancing circuits from a connected subscriber loop having a lengthshorter than a predetermined length, said enhancing circuits including avoice frequency bidirectional amplifier and supervisory signal detectingmeans.
 2. The telephone range extender according to claim 1 furthercharacterized bysecond means for detecting the central office voltageapplied to a connected subscriber loop, and means responsive to saidsecond detector means for detecting a central office voltage below apreselected magnitude for enabling said disconnecting means.
 3. Atelephone range extension system characterized bya plurality of rangeextenders each automatically adaptable to the length of a connectedtelephone subscriber loop, switching means for selectively connectingeach of said range extenders to any one of a plurality of subscriberloops, means for detecting the length of a connected subscriber loop,and means responsive to said detecting means for disconnecting saidrange extender from a subscriber loop having a length greater than apredetermined length.